Key takeaways
- The outdoor unit needs free air in and out, and a position that stops it drawing back its own discharge, because a starved coil loses performance and wears the equipment faster.
- Noise comes from the source, the path and the receiver, and a builder who only chooses a quieter model can still create a complaint by ignoring reflective surfaces and the neighbour’s bedroom window.
- The manufacturer’s clearances for intake, discharge and service are minimums, and a site that cannot meet them should be changed or documented rather than accepted by default.
- Condensate and frost must drain away from the building, the neighbour and any shared path, and the drainage should be designed before the pad is poured.
- A written record of the chosen position, the clearances and the reasons behind them is the builder’s best protection when a complaint arrives years after handover.
The outdoor unit is the part of a heat pump installation that gets the least thought at the design stage and the most attention after handover. It is usually the loudest piece of equipment on the property, it is often the most visible, and it is the one a neighbour will ring about on a warm night when the fan is running and the bedroom window is open. Owners tend to judge the whole system by it, whether or not the system is working well.
A heat pump does not make heat so much as move it. The outdoor unit draws air across a coil, takes heat out of that air, and discharges air that is cooler than the air that went in. How much heat it can collect depends on how freely air reaches the coil, how well the discharged air leaves the area, and whether the warm or cool air it has just handled comes straight back into the intake. The position on the wall or the pad decides all three, which is why placement is a performance question as well as a noise question.
This article covers the siting of the outdoor unit: airflow, noise, clearances, condensate and mounting. It does not cover choosing the size of the system or how much hot water a household needs, which sit in separate articles, and it does not cover the internal plumbing or the electrical circuit. Where the question is the hot water side of the same installation, our guide to heat pump hot water for builders is the better starting point.
Why placement decides how the system performs
Installers sometimes treat the outdoor unit as the last item on the list, to be put wherever the pipe run is shortest. That is backwards. The unit has to breathe, and every metre of pipe between the unit and the cylinder or indoor component is a cost that the owner pays over the life of the system through the energy it uses. Placement that starves the coil of air forces the unit to work harder, lowers the heat it can deliver and adds wear to the compressor and fan.
A unit that performs poorly is also a unit that gets blamed. When the hot water runs cooler than expected, or the system takes longer to recover after a long shower, the owner looks for a cause. The most common causes are not faults in the equipment. They are a unit boxed in by a fence and a wall, a unit sitting in the path of a dryer exhaust, or a unit drawing in air that has already been used once. Those are siting faults, and they are cheap to avoid at the design stage and expensive to correct later.
The builder’s job is to choose a position that gives the unit good air, keeps the discharge away from living areas and neighbours, and allows service access without a ladder and a crowbar.
Airflow: intake, discharge and the recirculation trap
The unit needs a clear path for the air it draws in, and a clear path for the air it throws out. The intake side needs open space in front of the coil so the fan is not pulling air from a narrow gap between two walls. The discharge side needs room for the air to leave without striking a wall, a fence or a window that sends it straight back toward the intake. Where the discharge is directed into a confined space, the cooled air can circulate back through the unit, and the unit slowly starts to work on its own exhaust.
This is the recirculation trap, and it is the reason corners are a poor choice. A unit tucked into the angle of a house and a boundary fence has its intake and discharge facing the same few square metres of air. In winter it cools that space further. In summer it heats the space it is already drawing from. Either way the coefficient of performance falls and the unit runs for longer. The manufacturer’s installation manual sets out the clearances that prevent this, and those clearances are the minimum, not a target.
Do not place the unit against the boundary without checking the neighbour. The mistake that costs the most is siting the unit against a shared fence, facing a neighbour’s bedroom window, because it was the only place the pipe run was short. The complaint, the relocation and the redone pad are paid for by the builder, and the unit may still fall short on airflow once it has moved.
Placement that performs and keeps the peace, against placement that causes complaints
| Placement that works | Placement that causes complaints |
|---|---|
| Open space in front of the coil and a clear discharge path | Tucked into a corner where the intake and discharge face the same air |
| Discharge directed away from living areas and the neighbour | Discharge aimed at a neighbour’s window or a boundary fence |
| Drainage falls away from the building, the path and the boundary | Condensate pools under the unit or runs onto a shared path |
| Isolated from the structure and reachable for service | Rigidly fixed to a light wall, or buried behind a tank and a gate |
Noise: the source, the path and the receiver
Noise from an outdoor unit has three parts, and each can be managed separately. The source is the compressor and the fan. The path is the air around the unit, the surfaces it bounces off, and the gap between the unit and the receiver. The receiver is the neighbour’s bedroom window, the living room on the other side of the fence, or the owner’s own sleeping area. A builder who thinks only about the source will buy a quieter unit and still get a complaint, because the sound is reflected off the fence and into the bedroom next door.
The source is partly fixed by the equipment. Manufacturers publish a sound rating for each model, and choosing a quieter unit is a legitimate step, though the rating is measured under the manufacturer’s test conditions and not in the street. The path is where the builder has most control. Hard, reflective surfaces close to the unit, such as a masonry wall or a metal fence panel, reflect sound. A unit set on a timber frame against a light cladding wall can transmit vibration into the building. Placing the unit further from the receiver, with a fence or a dense planting between them, helps more than most owners expect.
The receiver is the part nobody measures until it is too late. Before you fix the position, look at what the neighbour has built on the other side of the boundary. A bedroom with a window at the level of the unit is a different problem from a garage. Check the noise rules that apply where the property sits, because councils have their own expectations about what is acceptable at night, and a unit that complies on the day it is commissioned can still become a problem in summer when the fan runs for longer.
Clearances the manufacturer specifies, and why they exist
Every manufacturer publishes clearances for the outdoor unit. They cover the space needed around the unit for intake, for discharge, and for servicing, and they are set so that the unit can draw enough air and shed enough heat to run as designed. The builder’s obligation is to follow the manufacturer’s figures, which should be read from the installation manual for the model supplied, not from memory of a different brand or from a forum post.
Clearances are a minimum, and the reasons behind them are worth understanding so that a builder can reason about a site the manual never anticipated. The intake clearance keeps the fan from starving. The discharge clearance keeps the warm or cold air from being thrown back at the unit. The service clearance lets a technician reach the valves, the electrical connections and the coil for cleaning without moving the unit. Each of these matters on a job that has a tight block and a narrow side passage, which is where most of the problems are found.
When the site cannot meet the manufacturer’s clearances, the honest answer is to move the unit, not to hope. A builder who writes a note on the job file saying the clearance was not achieved, and why, has protected the client and the business. A builder who says nothing has created a fault that appears in two years and is blamed on the equipment. If a bracket or a raised platform is needed to achieve the clearance, specify it in the contract so the cost is not a surprise at the end of the job.
Condensate, frost and drainage
An outdoor heat pump unit makes water as it works. Moisture in the air condenses on the cold coil and drains from the base of the unit, and in cold weather the coil can frost and the unit goes through a defrost cycle that releases more water. The volume is not large, but it does not disappear. A unit set on a pad that falls back toward the house will drain into the footing and the wall. A unit set against the boundary drains into the neighbour’s side, and an icy patch on a shared path becomes a slip risk for a person who has nothing to do with the installation.
Plan the drainage before the pad is poured. The pad should fall away from the building and the unit should sit on a surface that lets the water run off rather than pool beneath it. Where the unit is raised on brackets or a platform, check that the condensate can leave the base and is not trapped by the bracket. In locations where frost is a real possibility, the position should avoid a spot where the water will freeze on a path or a step. The manufacturer will set out any drainage fitting the model needs, and that fitting should be installed as specified.
Mounting, vibration and access for service
A heat pump unit vibrates, and the vibration has to go somewhere. A unit fixed rigidly to a light wall or a timber deck can transfer its vibration through the structure and turn the building into a sounding board. The mounting should use the isolation the manufacturer calls for, which is usually a pad or a resilient mount between the unit and the base, and the base should be solid. A concrete pad is the usual answer. A timber platform is acceptable only if it is designed to carry the load and to avoid resonance, and the builder should not make that judgement without the manufacturer’s guidance.
Access for service matters more than most builders expect. The technician who visits in three years needs to reach the front panel, the valves and the electrical isolation without climbing over a fence or moving a water tank. A unit placed where a technician must stand on a wet step or lean over a gate is a unit that will be serviced less often, and a neglected coil loses performance quietly. Leave the space, label the isolator and record the position on the handover documents.
Working in heat on the install day
The outdoor unit is often installed in the hottest part of the year, because that is when the client has the most urgent need for hot water and the system is being replaced. The crew then works on a hot roof, against a hot wall, with a heavy unit and a long pipe run. Safe Work Australia’s guidance on working in heat sets out the duties of the person in control of the work, including the need to assess the conditions, provide water and shade, plan the heavy lifting for the cooler part of the day and watch for the early signs of heat stress.
Those duties apply to the installer on the job, and they are the builder’s responsibility where the builder is the person in control of the site. The commercial reality is that a crew working through the heat of the afternoon makes errors. Scheduling the outdoor work for the cooler parts of the day, and marking the clearances before the heavy lift, is both a safety measure and a quality measure.
Worked example: a builder in western Sydney and a late complaint
A builder in western Sydney replaced an electric storage system with a heat pump on a semi-detached house. The unit was placed in the narrow side passage, with the pipe run kept short and the fan facing the boundary fence. The crew finished the job, the client was pleased with the hot water, and the handover was signed off without a note on the position.
Some months later the neighbour on the other side of the fence complained that the unit ran through the night in summer and that the sound came straight into the bedroom. The complaint was not about a fault. The unit was working as designed, but it was sited for the pipe run, not for the airflow or the receiver. The discharge was pushing warm air into the same few square metres it was drawing from, so the unit ran longer and louder than it needed to. The client asked the builder to fix it.
The fix was expensive. The unit had to be relocated to the rear of the property with a new pad, a longer pipe run, a fence panel added to block the path of the sound, and a service isolator moved to a reachable position. The builder carried the cost of the relocation because the original position had been chosen without a note on the clearances or the receiver. The lesson the builder took into the next job was that placement is a decision to be recorded, not a detail to be left to the crew on the day.
A checklist before you fix the outdoor unit
Before the pad is poured or the bracket is fixed, the builder should be able to tick off a short set of checks. Does the position meet the manufacturer’s intake, discharge and service clearances, and is that written on the job file? Is the discharge directed away from living areas and from the unit’s own intake? Has the position been checked against the nearest neighbour’s windows and boundary? Does the condensate drain away from the building, the neighbour and any path? Is the mounting isolated from the structure as the manufacturer requires, and is the service access reachable without a ladder?
Before the pad is poured or the bracket is fixed
- Check the manufacturer’s intake, discharge and service clearances for the model supplied.
- Confirm the discharge is directed away from living areas and from the unit’s own intake.
- Check the position against the nearest neighbour’s windows and the boundary.
- Make sure the condensate drains away from the building, the neighbour and any path.
- Isolate the unit from the structure as the manufacturer requires.
- Confirm a technician can reach the valves and the isolator without a ladder.
- Record the chosen position and the clearances on the job file.
If any answer is no, the position is not ready. The cost of moving a unit before the pad is poured is a fraction of the cost of moving it afterwards, and the client will forgive a short delay far more readily than a neighbour’s complaint. Experienced builders treat the outdoor unit as a site decision with a record behind it, in the same way they treat the setback and the drainage on the house itself. The ventilation of the same space is covered in more detail in our article on heat pump dryers and laundry ventilation, which faces many of the same airflow questions.
What to tell the owner at handover
The owner should leave the site knowing where the unit is, why it is there, and what to keep clear of it. Tell them to keep the intake and discharge free of leaves, planting and stored items, and give them the manufacturer’s clearances in writing, so the next person who wants to build a pergola over it has the information.
The wider picture of building responsibly for the client and the neighbour is part of the professional standard expected of a licensed builder. For a broader view of the obligations that come with the licence, PointCPD’s guide to how NSW builder CPD works sets out how the requirements fit together.
Questions builders ask
Can an outdoor heat pump unit go against the boundary fence?
It can, but only if the intake and discharge clearances the manufacturer specifies are still met and the discharge does not reach a neighbour’s window or living area. Check the neighbour’s side before fixing the position, not after the first complaint. A position against the fence is often the most likely to create problems.
How do I stop a heat pump unit being too loud at night?
Start with the position, because the path and the receiver matter as much as the equipment. Keep the unit away from reflective surfaces that carry the sound, and use a fence or dense planting between the unit and the neighbour’s bedroom. Choose a model with a suitable sound rating, and check the noise rules that apply where the property sits.
Where should the condensate from the outdoor unit go?
Condensate should drain away from the building, from the neighbour’s property and from any shared path or step. Set the pad to fall away from the house and check that any bracket does not trap the water at the base of the unit. Follow the manufacturer’s drainage requirements for the model supplied.
Can I mount the unit on a timber deck?
A timber platform is only acceptable if it is designed to carry the load and to avoid transmitting vibration into the structure. Check the manufacturer’s guidance for mounting and isolation before using a deck. A solid base is usually the safer answer, and the choice should be recorded on the job file.
What if the site cannot meet the manufacturer’s clearances?
Move the unit to a position that does, or add a raised platform or bracket that achieves the clearance and specify it in the contract. Do not accept a position that falls short without writing the shortfall on the job file and telling the client. An undocumented shortfall usually surfaces as a performance or noise complaint later.
Who is responsible if the unit is placed badly and causes a complaint?
The builder who chose and fixed the position is usually the first person the client looks to, whatever the equipment supplier says. A written record of the position, the clearances and the reasons behind the choice is the best protection. Check your contract and insurance for how a relocation would be handled.
Sources
- Heat pump water heaters · energy.gov.au
- Working in heat · Safe Work Australia
Published 20 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
